1. Field of the Invention
The present invention relates to redundant devices that share configuration requirements. More particularly, the present invention relates to maintaining high availability of network devices.
2. Description of the Related Art
Within systems configured for using redundancy, at least two processing units are required where at least one is designated the “Active unit” while at least one other is designated the “Standby unit.” As used herein, a “unit” is one or more processors, line cards, etc. The role of the Standby is to take over for the Active unit should the Active unit fail, ideally without any service interruption. The Active unit is responsible for providing service and manages the configuration of the system. In order to provide the Standby capability, its view of the configuration must be kept synchronized with the view that the Active unit has at any point in time. Both the initial configuration and all subsequent changes to the configuration must be synchronized to the Standby unit.
In general, the system may be configured in a “cold,” “warm” or “hot” standby state. Although these terms are not precisely defined, an increase in figurative temperature indicates a relatively higher state of readiness of the Standby unit. In general, a cold standby state refers to hardware redundancy. In case of failure during a cold standby state the network device will reset, with the Standby unit becoming Active. In a warm standby state, there is hardware redundancy and enough application redundancy so that the Network Device doesn't have to reset on a switch over. However, sessions through the network device are lost and need to restart. A hot standby state means there is hardware redundancy and enough application redundancy that upon a switchover the device is not reset and sessions through the network device are maintained through the switchover. In the context of the current invention, we are describing configuration synchronization in a system operating in a hot standby mode known as “Stateful Switchover” (SSO).
The present assignee has developed methods wherein an Active unit may be executing an image that is different from that of the Standby unit. Among other things, these methods allow the software image of a network device's Standby unit to be changed while the network device is operating without any interruption of service. Such a change provides either a newer image (i.e., an upgrade) or an older image (i.e., a downgrade) to execute on the Standby unit. The system continues to operate in an SSO (hot standby) mode during this change.